The present invention relates to wireless networks and, more particularly, to a method of Multiple-Input-Multiple-Output (MIMO) communication among the nodes of such a network in which a node can use different sets of its antennas for different purposes, such as simultaneous transmission to two other nodes or simultaneous transmission and reception.
An ad hoc wireless network is a group of transceivers set together in an arbitrary formation with no permanent infrastructure. The transceivers form a network by contacting each other based on a predetermined set of protocols and establish the network in a distributed fashion. Conceptually, the network is a graph, with each transceiver being a node of the graph and the communication channels between the transceivers being edges of the graph. Consequently, the terms “transceiver”, “terminal” and “node” are used interchangeably herein. The network allows flexibility in the addition and subtraction of members.
In a network communications model analogous to the OSI seven-layer model of computer networking, the first layer of the network is based on its physical layer. This layer determines the frequency bandwidth, the modulation and the coding of the transmissions. The second layer of the model is the Medium Access Control (MAC) protocol. The MAC layer determines the timing and synchronization, and determines which of the users is allowed to access the physical medium and transmit at any given time, frequency and/or code. The MAC layer also prevents collisions that occur when multiple users attempt to use the same physical resource at the same time. The third layer of the model is the network layer. This layer determines the routes of transmission of messages according to predetermined algorithms based on predetermined metrics such as the length of the path, the connectivity, congestion, maximum flow, etc.
Future ad hoc wireless networks will be required to have improved performance compared to current wireless LAN standards such as IEEE 802.11b. Both the spectral efficiency and the channel utilization need to be improved. Recently, a wireless communications technique called VBLAST (P. W. Wolniansky et al., “V-BLAST: an architecture for realizing very high data rates over the rich-scattering wireless channel”, Proceedings URSI International Symposium on Signals, Systems and Electronics (IEEE, New York N.Y., USA, 1998) pp. 295-300) has been introduced to address the spectral efficiency challenge. VBLAST is a MIMO technique: it uses multi-element antennas at both the transmitter and the receiver to permit transmission rates far in excess of those possible using conventional approaches.
In wireless systems, radio waves do not propagate simply from transmit antennas to receive antennas, but bounce and scatter randomly off objects in the environment. This scattering is known as “multipath” because it results in multiple copies (“images”) of the transmitted signal arriving at the receiver via different scattered paths. In conventional wireless systems, multipath represents a significant impediment to accurate transmission, because the images arrive at the receiver at slightly different times and can thus interfere destructively, canceling each other out For this reason, multipath is traditionally viewed as a serious impairment. Using the VBLAST approach, however, it is possible to exploit multipath, that is, to use the scattering characteristics of the propagation environment to enhance, rather than to degrade, transmission accuracy by treating the multiplicity of scattering paths as separate parallel subchannels.
VBLAST accomplishes this by splitting a single user's data stream into multiple substreams and using an array of transmitter antennas to simultaneously launch the parallel substreams. All the substreams are transmitted in the same frequency band, so spectrum is used very efficiently. Because the user's data are being sent in parallel over multiple antennas, the effective transmission rate is increased roughly in proportion to the number of transmitter antennas used.
At the receiver, an array of antennas is again used to pick up the multiple transmitted substreams and their scattered images. Each receive antenna “sees” all of the transmitted substreams superimposed, not separately. But if the multiple scattering is sufficient, then the multiple substreams are all scattered slightly differently, because they originate from different transmit antennas that are located at different points in space. Using sophisticated signal processing, these slight differences in scattering allow the substreams to be identified and recovered. In effect, the unavoidable multipath is exploited to provide a very useful spatial parallelism that is used to greatly improve data transmission rates. Thus, when using VBLAST, the more multipath the better, just the opposite of conventional systems.
The VBLAST signal processing algorithms used at the receiver are the heart of the technique. At the bank of receiving antennas, high-speed signal processors look at the signals from all the receivers simultaneously, first extracting the strongest substream from the morass, then proceeding with the remaining weaker signals, which are easier to recover once the stronger signals have been removed as a source of interference. The ability to separate the substreams depends on the slight differences in the way the different substreams propagate through the environment.
VBLAST is only one of a set of related techniques for exploiting multiple scattering to obtain spatial parallelism. Other such techniques are described by B. Vucetic and J. Yuan in Space-Time Codes (Wiley, 2003) under the general heading of “layered space-time coding”. Examples of such techniques include, inter alia, the use of LST receivers, with or without parallel interference cancellation. These techniques are referred to herein generally as “spatial demultiplexing” techniques.
Ad hoc transmissions tend to be a mix of messages of various lengths. Examples of long messages include video streaming and VoIP services. Examples of short messages include routing messages, positions of mobile nodes, ACK messages when high reliability service is required, etc. While time division solves some of the bandwidth sharing problem, it is still inefficient to devote an entire time slot to a short message. This inefficiency is further increased when MIMO transceivers are used. For example, in VBLAST, a short message that spans, in its entirety, less than a single time slot, could occupy only a small part of the time required to send a larger message like a single image. Sending an ACK message or a routing message becomes intolerably wasteful, considering the overhead involved with each message transmission.
There is thus a widely recognized need for, and it would be highly advantageous to have, a MIMO wireless communication technique that preserves its inherent efficiency even when transmitting a short message.